The present invention relates generally to non-volatile memories, and more particularly to magnetic random access memories.
Magnetic Random Access Memory (MRAM) is a type of non-volatile memory with fast programming time and high density. The architecture for MRAM includes a plurality or array of memory cells and a plurality of word line and bit line intersections. Typically, the magnetic memory cell used is composed of a magnetic tunnel junction (MTJ) cell, an isolation transistor, and the intersection of word and bit lines. The isolation transistor is generally a N-channel field effect transistor (FET). An interconnect cell connects the isolation transistor to the MTJ device, to the bit line, and to the word line used to create part of the magnetic field for programming the MRAM cell.
Typically, a MTJ cell includes a non-magnetic conductor forming a lower electrical contact, a pinned magnetic layer, a tunnel barrier layer positioned on the pinned layer, and a free magnetic layer positioned on the tunnel barrier layer with an upper contact on the free magnetic layer.
The pinned layer of magnetic material has a magnetic vector that is always pointed in the same direction. The magnetic vector of the free layer is free, but constrained by the physical size of the layer, to point in either of two directions. An MTJ cell is used by connecting it in a circuit such that electricity flows vertically through the cell from one of the layers to the other. The MTJ cell may be electrically represented as a resistor, and the size of the resistance depends upon the orientation of the magnetic vectors. As is understood by those skilled in the art, the MTJ cell has a relatively high resistance when the magnetic vectors are pointed in opposite directions and a relatively low resistance when the magnetic vectors are aligned.
A bit line is generally associated with each column of an array of MTJ cells and a word line is associated with each row of the array. The bit lines and word lines are used to address individual cells in the array for both reading and programming or storing information in the array. Programming of a selected cell is accomplished by passing predetermined currents through the word and bit lines intersecting at the selected cell. For embodiment, the magnetic field resulting from a “write current” changes the direction of the data layer, or free layer. As a result, the data are written as soon as the “write current” is applied.
It is of course desirable to have the low resistance (parallel vectors) as low as possible, and the high resistance (anti-parallel vectors) much higher than the low resistance so that the change can be easily detected in associated electronic circuitry.
Desirable in the art is an improved high density MRAM device.